Ropes, cables and the like are used extensively for many purposes such as ropeways, cable cars, ski lifts, chairlifts, elevators, and military applications, but are of particular importance in the mining industry where the Intent is to use these for raising and lowering conveyances carrying personnel, equipment, material, waste and ore in underground operations, such as between the mining accesses and the surface. For such applications, ropes may have considerable length and must carry considerable loads, including the weight of the ropes themselves in the sections between the conveyances and the mine hoists at the surface (and underground as well) and used for their deployment.
One of the key concerns for all ropes is to determine when the rope is still in safe working condition or should be replaced. The costs to replace ropes can be very significant, and yet timely replacement is imperative to avoid excessive rope wear and rupture. To ensure operational safety and acceptable operational life of the system, the physical condition of such ropes must be monitored frequently, for example as required by specific regulations. For this reason, since industrial applications typically utilize wire ropes, previous efforts have focused upon devices and methods to test wire ropes for potential wear or deterioration. Some of these devices and methods have enabled on-site testing of the rope whilst in situ at the point of use, without causing damage or destruction to the rope. Such devices and methods are particularly advantageous as they minimize the impact of costly operational disruptions and stoppages. Wire ropes or cables and the like may thus be retained in situ for continued use (with periodic testing) until their safe operational life is substantially completed, for example if the rope parameters fall outside of regulatory requirements.
One important parameter to assess wire or synthetic rope condition, but not exclusively, is to test for “lay length”. For example, wire ropes are made up of twisted or braided metal wires. Individual metal wires are twisted together to form bundles or strands, and then a number of such strands are twisted together to form a rope or cable. The lay length of such a rope is the distance along the rope (measured parallel to the centre line or axis of the rope) in which a strand at or beneath the surface makes one complete turn or helical spiral around or within the rope. Often, the lay length is measured over a few lay lengths and then the measurement is divided by the number of lay lengths to produce an average lay length value over the measured section. The lay length is known when the ropes is first manufactured (or at least after the strands have been allowed to settle into their more-permanent positions following a few lifting cycles) but it will change during use. For example, in mining applications the lay length changes with depth due to the torsional behavior of stranded hoist rope. These variations evolve over the life of the rope and must be monitored to ensure that they remain within established operational or safety parameters. Localized faults, wear, corrosion, core deterioration, strand breakage etc. may all cause increased lay length. The relevance of changes in lay length of a rope can require expert interpretation and/or precise monitoring. In general, if the lay length of a rope or cable and the like changes beyond defined limits, or if it changes locally, this may indicate potential failure of the rope, and the requirement for rope replacement.
Various testing methods are known for assessment of ropes. For example, in magnetic field testing a wire rope is brought into a magnetic field, and the presence of defects in the wire rope is detected through areas of induced flux changes. In other examples, eddy current testing comprises passing an alternating electrical current through a coil producing a magnetic field. When the coil is placed near a conductive material, the changing magnetic field induces closed loops of current flow known as eddy currents in the material, which produce their own magnetic fields that can be measured and used to determine the presence of flaws in the wire rope.
Synthetic ropes are in principle attractive for the replacement of wire ropes in numerous applications because they have a number of advantages over wire ropes including: higher strength to weight ratios, corrosion resistance, better fatigue life, and lower maintenance requirements. However, compared to wire ropes, it can be more difficult to assess local faults as well as the lay length of synthetic ropes as they are typically comprised of non-metallic substances not amenable to the aforementioned magnetic field techniques. Often, those testing or monitoring of synthetic ropes must rely upon visual inspection, or Imaging techniques to assess rope wear and integrity, which may be less reliable and may fail to provide an accurate assessment of broken strength-member fibres, lay length and/or rope condition. The problems associated with such inspection techniques may be further exacerbated by the use of non-load-bearing covers, which are sometimes applied to synthetic ropes to protect the strength member fibres of the synthetic rope from damage and/or UV radiation, but which otherwise obscure the strength-member fibres from visual inspection.
Thus, there remains a need in the art for devices and methods for analysis of synthetic ropes and cables. More particularly, the need extends to assessment of lay length of synthetic ropes and cables, and/or assessment of wear or damage including breakage of strength-member fibres of synthetic ropes or cables.